REWORKABLE ADHESIVE FOR ELECTRONIC APPLICATIONS

Description

Field of Invention

[0001] The present invention relates to an adhesive composition, and particularly to an adhesive composition that is reworkable or repairable in use with an integrated circuit, rigid circuit, or flexible circuit.

Background Art

[0002] Electronic devices, such as calculators, watches, portable computer terminals, and "smart" cards, utilize a wide variety of integrated circuits, rigid circuits, and flexible circuits. In such devices, there are a number of means used for attaching and providing electrical connection between a circuit and a conductive substrate. The means for providing attachment and electrical connection include metallic solders and adhesive compositions, including conductive adhesive compositions.

[0003] Conductive adhesives may contain sparse and randomly dispersed populations of fine conductive particles. The particles provide electrical conductivity through the thickness of the adhesive, but not in the plane of the adhesive film. Such adhesives are sometimes provided as an adhesive film, often referred to as Z-axis films (ZAF).

[0004] Adhesives that are used in known Z-axis adhesive films typically have glass transition temperatures (T_g) of less than about 50°C, and are usually soft, rubbery, thermoplastic materials having little or no crosslinking. The use of such an adhesive permits rapid bonding, at a modest temperature, with ease of repairability.

[0005] A repairable, or reworkable, adhesive is an adhesive that can be entirely removed from the bonded device with heat and/or solvent. The use of a reworkable adhesive allows for the bonded device to be removed and repositioned without damage to the device, e.g., a printed circuit or glass display. Displays or boards containing more than one chip are expensive. If one chip is defective, it is highly desirable to be able to replace it.

[0006] A deficiency in soft, rubbery, thermoplastic adhesives used for electrical connection is that they typically have poor creep resistance at high stress and/or temperature. Accordingly, such adhesives are useful only in applications involving limited stress. Thus, such adhesives are almost exclusively used for the bonding of lightweight, flexible circuits to other components, an application with relatively minimal stress. An example of such a thermoplastic adhesive for use as a repairable adhesive film in electronic applications is disclosed in U.S. Patent No. 4,820,446 (Prud'Homme).

[0007] While certain highly crosslinked or thermoset adhesive systems perform adequately in high stress applications, these adhesives are deficient in that they are not reworkable. An example of such a non-reworkable adhesive composition, exhibiting superior shear strengths at high temperatures, is disclosed in U.S. Patent No. 4,769,399 (Schenz).

[0008] An application in the electronic industry with particularly demanding performance requirements has arisen, at least in part, from the demand for increased power to size ratio in electronic products, and the resultant need for higher input/output (I/O) counts and I/O densities between components. In addition, the demand for higher information flow rates has necessitated higher signal frequencies. To accommodate these needs, Flipped Direct Chip Attachment (FDCA) is used to attach a chip directly to a circuit board, and provide the shortest attainable path length between components, thus minimizing signal propagation delays at high frequency.

[0009] The most common attachment means used in FDCA is solder bump/flip-chip interconnection. With this approach, metallurgical solder joints provide both the mechanical and electrical interconnections between the chip and the substrate. This method has inherent pitch limitations and also is susceptible to mismatches between the coefficients of thermal expansion (CTE) of the chip and the substrate. Substantial CTE mismatch between these materials results in high shear stresses in the solder joints which can compromise the reliability of the interconnections. See, R.R. Tummala and E.J. Rymaszewski, Microelectronics Packaging Handbook, (Van Nostrand Reinhold, 1989) pp. 280-309; 366-391; and K. Nakamura, Nikkei Microdevices, June 1987. Catastrophic failure is the immediate result of any cracking that occurs in these solder joints as a reaction to these stresses.

[0010] Because of the inherent deficiencies with solder bump bonding, an alternate means of interconnection for FDCA and other applications is highly desirable. One method involves the use of a heat-bondable adhesive, which may or may not be curable, to provide an intimate mechanical flip-chip bond, and to provide pressure engaged, rather than metallurgical, electrical interconnections to the substrate. Conductive particles within the adhesive or, alternately, metallic bumps on the chip itself, provide the electrical interconnection media for this method. Thermal and/or cure shrinkage stresses in the adhesive can be used to provide the compressive forces on the interconnection media that are needed in order to establish the pressure engaged contacts. In order to permit the buildup of contacting forces on the interconnection media, it is necessary that the interconnection media remain in the solid state during the bonding operation.

[0011] Pressure engaged interconnections are more able to accommodate shear stresses than are solder joints. Also, the presence of an adhesive at the bond line will tend to dramatically reduce the magnitude of the shear stresses at the contact sites. Therefore, CTE mismatches are better accommodated by FDCA with adhesives than by solder bump bonding.

[0012] Another potential advantage of FDCA with adhesives arises from the fact that the interconnection media (i.e., conductive particles in the adhesive or metallic bumps on the chip) do not melt during the bonding process, as does, for example, solder. It is much easier to control pad-to-pad spacing when the conductive structures in the bond line remain in the solid state during processing. Because of this, FDCA with adhesives provides the potential for reduced, and also more precise, spacing between interconnections as compared to solder bump bonding.

[0013] An important obstacle preventing the use of adhesives for FDCA has been the lack of an adhesive that has adequate creep resistance at elevated temperature and stress to provide stable interconnections and that also provides ease of handling and repairability. After bonding, the adhesive layer is highly stressed in tension. In order to provide a stable interconnection, the adhesive layer must be able to sustain this internal stress without creeping or fracturing. The high stress is the result of: a) the adhesive layer being as much as twenty times thinner than either a silicon chip or a rigid printed circuit board; and b) the very high CTE and very low elastic modulus of adhesives relative to either of these substrates.

[0014] The magnitude of the tensile stress in the adhesive layer is sufficiently high that known thermoplastic resins that are heat processable at temperatures of less than 200°C are unable to provide stable interconnections over the required range of operating temperatures. Highly crosslinked thermoset resins can be found that will provide stable interconnections at these high stresses and temperatures, but those materials are not reworkable with non-destructive methods.

[0015] In addition to the thermoplastic and thermoset adhesive systems, thermoplastic/thermoset blends are of possible interest for electrical interconnections. Such mixtures have been designed to improve high temperature performance of the thermoset materials, and/or to improve the fracture toughness of the thermoset material. See, U.S. Patent No. 3,530,087 (Hayes et al.); and R.S. Bauer, Toughened High Performance Epoxy Resins: Modifications with Thermoplastics, 34th International SAMPE Symposium, May 8-11, 1989.

[0016] However, despite the prior uses of thermoplastic adhesives, thermoset adhesives, and mixtures thereof, presently known interconnect means for demanding applications, such as FDCA, have failed to adequately solve the problems set forth above. Accordingly, there remains a need for a reworkable adhesive material that permits rapid bonding at a modest temperature and that also has a modulus sufficient to withstand significant stress and/or high temperatures, at least through the range of use temperatures. The adhesive should also be reworkable at a processing temperature, T_p, that is sufficiently low so that the substrate is not damaged during removal of a chip. There is also a need for such an adhesive that has an extended shelf life at room temperature, has a low viscosity at the intended bonding temperature to provide good flow properties, is resistant to conditions of up to 85°C and 85 percent relative humidity, is resistant to higher qualifying temperatures, and may be provided as a film that is substantially tackfree at the intended handling temperature.

Disclosure of the Invention

[0017] The present invention provides a reworkable adhesive film that permits rapid bonding at modest temperatures, while having sufficient storage modulus at low frequencies, such as 1 hertz, (hereinafter "modulus") to provide significant contacting forces over the range of temperatures useful in electronic applications. The adhesive provides a sufficient modulus through a range of use temperatures, but is also reworkable at a suitable processing temperature. The present invention also provides such a reworkable adhesive that has an extended shelf life at room temperature, has a low viscosity at the intended bonding temperature and pressure to provide good flow properties, is resistant to conditions of up to 85°C and 85 percent relative humidity, is resistant to conditions up to higher qualifying temperatures (defined below), and may be provided as an adhesive film that is substantially tackfree at typical handling temperatures.

[0018] The present invention provides a reworkable adhesive composition, preferably provided as a nontacky or slightly tacky adhesive film, comprising a blend of:

a) a thermoplastic polymer having a T_g of 30°C to 250°C, wherein said thermoplastic polymer is selected from polysulfones, polyamides, polyesters, polycarbonates, polyvinyl acetals and polyethers;

b) a crosslinkable resin having a T_g of 125°C to 300°C;

c) a catalyst capable of accelerating the cure reaction of said crosslinkable resin, and

d) conductive particles;

wherein said adhesive composition is curable to a reworkable, electrically conductive Z-axis adhesive.

[0019] The cured adhesive composition preferably has a shear strength of less than about 1 megaPascals ("MPa"), more preferably less than about 0.5 MPa, at a temperature of about 20°C above the T_g of the cured adhesive composition. The adhesive composition preferably has such a shear strength at the processing temperature, T_p, of the cured adhesive composition. T_p is typically in the range of about 125°C to 250°C, and preferably in the range of about 150°C to 200°C.

[0020] The cured adhesive composition preferably has a modulus of greater than about 1 x 10² MPa, more preferably greater than about 1 X 10³ MPa, at a temperature of about 20°C below the T_g of the cured adhesive composition. The adhesive composition preferably has such a modulus at the qualifying temperature, T_Q, of the adhesive composition. T_Q is typically in the range of about 80°C to 150°C, and preferably in the range of about 100°C to 125°C. T_Q is less than T_P.

[0021] The crosslinkable resin has a T_g of about 125°C to 300°C. The cured adhesive composition preferably has a T_g of about 100°C to 200°C.

[0022] Where a T_g value is given for a crosslinkable resin or adhesive composition it refers to the T_g of the material in its polymeric or crosslinked form. As the material may comprise one or more crosslinkable and noncrosslinkable moieties, the material, from which T_g is reported, may be a homo-, co-, ter-, or oligopolymer. Some materials may demonstrate more than one T_g. In such a case, T_g will refer to the higher or highest of the T_g values for the material.

[0023] The crosslinkable resin preferably comprises an epoxy resin or a cyanate ester resin. The epoxy resin preferably comprises an aromatic epoxy, and may also include an epoxy curative. The epoxy resin may typically comprise one or more components individually having a molecular weight of about 200 to 2000. The cyanate ester resin may typically include one or more components individually having a molecular weight of about 150 to 2000.

[0024] The epoxy resin preferably includes one or more epoxy compositions according to Formulas I, II, and/or III below:

where X is a single bond, a C₁-C₄ alkylene group, or the "-SO₂- group":

"alkylene or alkyl groups", as used herein, includes substituted alkylene or alkyl groups, such as halogenated alkylenes or alkyls (e.g., fluorinated alkylenes or alkyls); X is preferably a single bond, -CH₂-, -C(CH₃)₂-, or the -SO₂- group; and R¹, R², R³, R⁴, R⁵, R⁶ are each independently -H, a C₁-C₅ alkyl group, or the "Epoxy Group":

with at least two of R¹-R⁶ being the Epoxy Group; preferably R² and R⁵ are both the Epoxy Group, and R¹, R³, R⁴, R⁶, are independently -H, or -CH₃;

where R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹² are independently -H, a C₁-C₅ alkyl group, or the Epoxy Group, and k is 0 or 1, m is 0 to 9, with at least two of R⁷-R¹² being the Epoxy Group, except when k is 0, wherein both R⁷, R⁸ are the Epoxy Group. R⁷-R¹² are preferably independently -H, -CH₃, or the Epoxy Group, with at least two of R⁷-R¹² being the Epoxy Group; and

where R¹³ , R¹⁴ are independently

where R¹⁵, R¹⁶, R¹⁷ are independently -H, a C₁-C₅ alkyl group, or the Epoxy Group, preferably -H, -CH₃, or the Epoxy Group, where R¹³, R¹⁴ combined include two or more Epoxy Groups; R¹⁵-R¹⁷ may be different for R¹³ and R¹⁴.

[0025] The epoxy curative is preferably a fluorene curative according to Formula III, except that one of R¹⁵-R¹⁷ is -NH₂, or -NHCH₃, and the other two of R¹⁵-R¹⁷ are independently -H, or a C₁-C₃ alkyl group; R¹⁵-R¹⁷ may be different for R¹³ and R¹⁴.

[0026] The cyanate ester resin preferably includes one or more cyanate ester compositions according to Formulas IV, V and/or VI below:

where Y is a single bond, a C₁-C₄ alkylene group, -S-, or the -SO₂- group; and where R¹⁸-R²³ are independently -H, a C₁-C₅ alkyl group, or the Cyanate Ester Group:

        -OC≡N

with at least two of R¹⁸-R²³ being the Cyanate Ester Group. R¹⁸-R²³ are preferably -H, -CH₃, or the Cyanate Ester Group;

where n is 0 to 5; and

where R²⁴, R²⁵ are independently

where R²⁶, R²⁷, R²⁸ are independently -H, a C₁-C₅ alkyl group, or the Cyanate Ester Group, preferably -H, -CH₃, or the Cyanate Ester Group, where R²⁴, R²⁵ combined include two or more Cyanate Ester Groups. R²⁶-R²⁸ may be different for R²⁴ and R²⁵.

[0027] The thermoplastic polymer preferably has a molecular weight of about 3,000 to 200,000. Lower molecular weights are preferable to promote resin flow at the bonding temperature. The thermoplastic polymer is selected from polysulfones, polyvinyl acetals, polyamides, polyesters, polycarbonates, and polyethers.

[0028] The present invention also provides a method for forming a reworkable adhesive bond between two conductive surfaces to form a conductive bonded composite. The method comprises the steps of:

a) providing a film of reworkable electrically conductive Z-axis adhesive composition of the present invention,

b) providing a conductive substrate (10) having one or more conductive bonding sites (20, 30, 40) ;

c) providing one or more conductive devices (50, 60-66, 80, 90) each having a conductive bonding surface;

d) positioning said reworkable adhesive between each of said conductive bonding surfaces of said conductive devices; and

e) heating to a temperature of from 125°C to 250°C for from 5 to 300 seconds to form a reworkable adhesive bond between each conductive bonding site and each conductive bonding surface.

[0029] Each of the reworkable adhesive bonds preferably has a shear strength of less than about 1 MPa at a temperature of about 20°C above the T_g of the cured reworkable adhesive film, and preferably at T_p of the adhesive composition. The reworkable adhesive preferably has a modulus of greater than about 1 X 10² MPa at a temperature of about 20°C below the T_g of the cured adhesive film, and preferably at T_q. The conductive substrates are typically printed circuit boards having conductive bonding sites adapted to be bonded to integrated circuits, flexible circuits, rigid circuits, or the like.

[0030] "Reworkable", as used herein, is defined as an adhesive that is entirely removable from the substrate at a temperature of less than about 250°C, preferably less than 200°C. A reworkable adhesive will typically be entirely removable at a temperature of about 10°C to 30°C above the T_g of the adhesive. Higher temperatures may be used to further facilitate removal in many instances, but excessively high temperatures may degrade the substrate or conductive devices thereon. Any residue remaining on the substrate can be removed by rubbing with a porous, non-abrasive implement, such as a cotton swab, saturated with a suitable solvent, e.g., methylene chloride, tetrahydrofuran, or acetone. It is preferred that the reworkable adhesive film be removable from a substrate selected from : conductive materials such as copper, gold, silver, aluminum, nickel, and solder; dielectrics such as ceramic, glass, silicon and epoxy/glass laminates; and polymer films such as polyimide and PET, with application of a shear force of less than about 1 MPa.

[0031] "Effective amount", as used herein, is defined as an amount of a component, that when blended with the other components, is sufficient to provide a reworkable adhesive, preferably providing an adhesive having a shear strength of less than about 1 MPa at a temperature of about 20°C above the T_g of the cured adhesive composition, and a modulus of greater than about 1 x 10² MPa at a temperature of about 20°C below the T_g of the cured adhesive composition. An adhesive including "an effective amount" of each of its components will preferably be removable with a shear strength of less than about 1 MPa at T_P.

[0032] "Crosslinkable resin", as used herein, is defined as one or more moieties that react to form a crosslinked network.

[0033] "Thermoplastic polymer", as used herein, is defined as a polymeric component that does not react with the crosslinkable resin to an appreciable degree, and the use of the term "reaction product", referring to the cured adhesive composition, is not meant to indicate that there is reaction between thermoplastic polymer and crosslinkable resin.

[0034] An effective amount of the epoxy resin including curative, if any, is typically 25 to 80 percent by weight of the total adhesive resins in the adhesive film, and preferably about 40 to 65 percent by weight of the adhesive resins. An effective amount of the cyanate ester resin is typically about 25 to 75 percent by weight of the adhesive resins, and preferably about 40 to 65 percent by weight of the adhesive resins. Adhesive resins refers to the thermoplastic polymer and crosslinkable resin, excluding any solvent, optional fillers, etc. that may be contained in the total adhesive film.

[0035] The subject invention also provides an electronic assembly, comprising:

a) a first electronic component (10, 20, 30, 40, 50, 60, 61-66, 70, 80, 90, 100) with a conductive bonding site;

b) a second electronic component (10, 20, 30, 40, 50, 60, 61-66, 70, 80, 90, 100) with a conductive bonding site;

c) the adhesive composition of the subject invention between a bonding site on the first component and a bonding site on the second component.

Brief Description of the Drawings

[0036] The invention will be further explained with reference to the drawing, wherein:

Figure 1 depicts a plot of temperature (X-axis) versus modulus (Y-axis) for three adhesive compositions including an adhesive composition of the present invention;

Figure 2 is a schematic plan view of integrated circuits, printed circuit boards, and the like on which the adhesive composition of the present invention can be used;

Figure 3 is a graph depicting thermal cycling for Examples 1-6;

Figure 4 is a graph depicting thermal aging for Examples 1-6;

Figure 5 is a graph depicting the environmental testing of Example 7; and

Figure 6 is a graph depicting the environmental testing of Example 8.

Detailed Description of Illustrative Embodiments

[0037] "Shear strength" as used herein refers to the force needed to shear a chip off a glass substrate as illustrated in Example 9.

[0038] A reworkable adhesive composition of the present invention may be produced by combining an effective amount of a thermoplastic polymer having a T_g of about 30°C to 250°C, with an effective amount of a crosslinkable resin. Thus, the adhesive film has a sufficiently high shear strength (above τ_U in Figure 1) throughout the desired range of use temperatures (T_Q and below in Figure 1), but demonstrates a sufficiently rapid drop in shear strength above its T_g so that at a processing temperature, T_P, the adhesive is reworkable.

[0039] The reworkable adhesive film of the present invention is also preferably storage stable for at least one month at room temperature, has a sufficiently low viscosity at the bonding temperature to provide good flow properties, is rapidly bondable at a modest temperature, and, after bonding is not substantially affected by conditions of up to 85°C at 85 percent relative humidity, or temperatures of up to T_Q.

[0040] The reworkable adhesive of the present invention is particularly useful in electronic applications. Many types of electronic components include one or more silicon chips. These chips are normally rectangular or square in shape and are of small dimension, e.g., a few millimeters (mm) on a side. Each chip has at least one, and often many, conductive terminals that must be electrically connected to conductive pathways on a substrate in order to complete the desired circuit. The pathways may comprise thin films or strips of metal, e.g., silver, gold, copper, etc., bonded to the substrate, or may consist of strips of an adhesive or ink which is electrically conductive or rendered electrically conductive by the presence of conductive particles therein.

[0041] The reworkable adhesive of the present invention is particularly useful as an attachment means in the FDCA bonding method. The FDCA bonding method bonds a bare chip directly to a conductive substrate. Certain chips have bumps (bumped chips) adapted to bridge the adhesive layer and connect directly to the substrate. An unbumped chip is typically bonded using an adhesive having an array of conductive particles that will form conductive pathways between the substrate and the chip. FDCA bonding is discussed in P.B. Hogerton et al., Investigations Into The Use Of Adhesives For Level-1 Microelectronic Interconnections, Electronic Packaging Materials Science, Symposia Proceedings, April 24-29, 1989. It has been observed that reworkable adhesive compositions of the invention containing epoxies with fluorene amine curatives as the crosslinkable component have exhibited better environmental performance in FDCA applications than those containing cyanate ester crosslinkable components.

[0042] The range of typical commercial bonding temperatures is in the range of T_p, about 125°C to 250°C, and preferably about 150°C to 200°C. Bonding times can range from a few minutes to a few hours in the use of prior art adhesives, however, for a commercially feasible, mass production assembly of an electronic component, it is preferred that the bonding time be about 5 seconds to 5 minutes, preferably about 5 to 180 seconds.

[0043] The reworkable adhesive composition of the present invention is preferably provided as a film that is nontacky or slightly tacky at the intended handling temperature. The advantages of such an adhesive film over a liquid or paste adhesive include ease of handling, accurate alignment of the chip, and the ability to place the exact amount of adhesive onto the chip to be bonded with minimal waste.

[0044] Referring to Figure 1, an illustrative plot of temperature (X-axis) versus shear strength (Y-axis) for three general types of adhesive compositions is shown. Adhesive 1 is a thermoplastic adhesive, Adhesive 5 is a thermoset adhesive, and Adhesive 3 is an adhesive of the present invention. T_Q, or the qualifying temperature, represents the upper end of the desired range of use temperatures. For many intended uses there are set or standard qualification temperatures at which adhesives are tested to determine whether they qualify for a given application. Thus, temperatures below T_Q, falling within the range of use temperatures, are those temperatures at which the adhesive is expected to be subjected to in use. T_Q is typically about 80°C to 150°C, more typically about 100°C to 125°C.

[0045] The processing temperature, T_P, is the temperature at which the adhesive will be used to bond the chip to a substrate. T_P must be sufficiently low so that the chips or substrates are not damaged during processing, and must be sufficiently high so that the adhesive will adequately bond the chip to the substrate. T_P is typically about 125°C to 250°C, more typically about 150°C to 200°C. The reworkable shear strength, τ_R, is the maximum shear strength at which the adhesive is reworkable. Thus, it is desirable that the shear strength of the adhesive at T_P be below τ_R. The use shear strength, τ_U, is the required shear strength for adequate performance of the adhesive in its intended use.

[0046] As shown in Figure 1, Adhesive 5 (thermoset) has a high shear strength through the range of use temperatures, but is not reworkable. The Adhesive 1 (thermoplastic) has a sufficiently low shear strength so that it is reworkable, but does not have a sufficient shear strength throughout the range of use temperatures. Adhesive 3, an adhesive of the present invention, has a shear strength above τ_U below T_Q, and has a shear strength below τ_R at T_P.

[0047] Figure 2 is a schematic plan view of integrated circuits, printed circuit boards, and the like, on which the adhesive composition of the present invention is particularly suitable for use. A printed circuit board 10 is shown having flexible circuit terminals 20, printed circuit board terminals 30, and printed circuit board terminals 40. An adhesive of the present invention may be utilized, for example, to bond flexible circuit 50 to the flexible circuit terminals 20; a printed circuit board 60 to the printed circuit board terminals 30; and a TAB, i.e., tape automated bonding, lead frame 70 to the printed circuit board terminals 40.

[0048] A TAB lead frame 72 is shown bonded to the printed circuit board 10. The printed circuit board 60 has flip chips 61-66. Flip chip 66 is shown prior to bonding to the printed circuit board 60. An adhesive of the present invention could be used to bond flip chips 61-66 to printed circuit board 60. Also shown in Figure 2 is a liquid crystal display 80. The liquid crystal display 80 has the flexible circuit 50 bonded thereto. A flip chip 90 and a TAB lead frame 100 are also shown bonded to the liquid crystal display 80.

[0049] Figure 3 contains six graphs which illustrate the stability of interconnection resistances throughout 1000 hr exposures to thermal cycling between -55°C and 125°C. The interconnect pads on all of the test chips were located around the perimeters of the chips, and the graphs depict the distribution of resistances as a function of position about the perimeter. Each line on each graph represents the resistance distribution at a single point in time. Data has been included in each graph representing 0 hr, 150 hr and 1000 hr check points. In some cases, additional data at the 500 hr checkpoint is included as well. Each graph represents a different combination of chip type and substrate described below.

[0050] Referring to Figure 3, Samples A, B and C utilized glass test substrates with Indium-Tin Oxide (ITO) test circuits having a sheet resistance of 20 ohms per square. Samples D, E, and F utilized glass test substrates with nickel test circuits having a sheet resistance of less than 1 ohm per square. Samples A and D utilized test chips (3M120 test chips) with dimensions 6.73 x 6.73 mm and having 120 gold bumped pads with a center to center spacing of 0.2 mm. Samples B and E were similar to Samples A and D but utilized test chips which had unbumped pads. Samples C and F were similar to samples A and D but utilized test chips (3M328 test chips) with dimensions 8.76 x 8.76 mm and having 328 gold bumped pads with a center to center spacing of 0.1 mm.

[0051] Additional environmental aging results for flipped direct chip attachment to glass are shown in Figure 4. Configurations G-L correspond exactly with configurations A-F from Figure 3, but were aged at 125°C for 1000 hrs.

[0052] Referring to Figure 5, a graph depicting environmental testing of Example 7 (samples M-P) is shown. Figure 5 depicts environmental testing of a flex circuit having 0.4 mm center to center pad spacing bonded to a corresponding circuit board and having failure resistance equal to 10 ohms. Sample M was tested at 85°C and 85 percent relative humidity. Sample N was tested at 60°C, 95 percent relative humidity and 15 volts. Sample O was tested at 125°C. Sample P was tested at -55°C to 125°C.

[0053] Referring to Figure 6, a graph depicting the environmental testing of Example 8 (samples Q-T) is shown. The tests depicted in Figure 6 were conducted in a similar manner to those of Figure 5.

Thermoplastic Polymer

[0054] The adhesive composition of the present invention includes a thermoplastic polymer in an amount sufficient to render the adhesive composition reworkable. The crosslinkable resin provides a crosslinked structure to the adhesive sufficient to provide an adequate modulus and shear strength over the desired range of use temperatures and to prevent creep. The thermoplastic polymer is integrated with the crosslinked polymer to render the total adhesive composition reworkable at T_P, the processing temperature.

[0055] A thermoplastic polymer or resin is typically present at about 20 to 75 percent by weight.

[0056] In Figure 1, a plot of shear strength versus temperature is provided. The reworkable adhesive of the present invention, Adhesive 3, maintains a sufficiently high shear strength throughout the range of use, but the shear strength drops sharply above the T_g of the adhesive to provide a shear strength of less than about 1 MPa at T_P, which may correspond to a temperature of about 20°C above the T_g of the cured adhesive film. This allows, for example, a bonded chip to be removed from the substrate at T_P. Remaining adhesive residue may be removed by rubbing with a porous, non-abrasive implement, such as a cotton swab, that has been saturated with a suitable solvent.

[0057] Suitable thermoplastic polymers include polysulfones, polyvinyl acetals, polyamides, polyesters, and polycarbonates.

[0058] The use of polysulfones and polyvinyl acetals has proven particularly useful in applications involving FDCA, and are thus preferred.

[0059] Suitable polysulfones include those falling within Formula VII below.

wherein m is 0 or 1, and n is 10 to 500. When m is 0, n is preferably about 12-50, and when m is 1, n is preferably about 40-70.

[0060] Examples of suitable polysulfones include P1700-NT11, a polysulfone resin from Amoco Performance Products; and Victrex PES 5003P, a polysulfone, commercially available from ICI Advanced Materials.

[0061] Suitable polyvinyl acetals include those falling within Formula VIII below.

wherein X is -H or a C₁-C₄ alkyl group, and m is 80 to 2400, n is 10 to 2200, and p is 0 to 500. Preferably m is greater than n, n is greater than p, m is less than 800, and each monomer is randomly distributed.

[0062] Examples of suitable polyvinyl acetals include polyvinyl butyrals, such as Sekisui SLEC BX-L.

[0063] Polyamides include Unirez 2636, commercially available from Union Camp. Polyesters include Dynapol 206, commercially available from Hüls America, Inc. Polycarbonates include GE R3320.

Crosslinkable Resins

[0064] The adhesive composition of the present invention also includes an effective amount of a crosslinkable resin. The crosslinkable resin includes one or more moieties that react to form a crosslinked network. Preferred crosslinkable resins include: epoxy resins, comprising one or more epoxy compounds, optionally including an epoxy curative; and cyanate ester resins, comprising one or more cyanate ester compounds. The epoxy resin preferably includes one or more aromatic epoxy compounds. Aromatic epoxies typically provide adhesive compounds having a higher shear strength than do aliphatic epoxies. The epoxy resin will typically be present at about 25 to 80 percent by weight, and it is preferred that the epoxy resin be present at about 40 to 65 percent by weight. An epoxy curative may also be utilized as required, and is considered to be a part of the epoxy resin.

[0065] The epoxy resin preferably comprises epoxies according to one or more of the Formulas I, II, and/or III, as set forth above. The epoxy curative preferably falls under Formula III except that one of R¹⁵-R¹⁷ is -NH₂ or -NHCH₃, and the remaining two are independently -H, -CH₃ as set forth above.

[0066] Examples of suitable fluorene epoxy curatives are described in U.S. Patent No. 4,684,678, and include 9,9-bis(4-aminophenyl)fluorene; 9-(3-methyl-4-aminophenyl)-9-(3'-methyl-4'-methylaminophenyl)fluorene; 9,9-bis(4-methylaminophenyl)fluorene; 9-(4-methylaminophenyl)-9-(4'-4-methyl-9,9-bis(4-methylaminophenyl)fluorene; aminophenyl)fluorene; 9,9-bis(3-methyl-4-aminophenyl)fluorene; 9,9-bis(3,5-dimethyl-4-aminophenyl)fluorene; and 9-(3,5-dimethyl-4-methylaminophenyl)-9(3',5'dimethyl-4'aminophenyl)-fluorene.

[0067] Examples of suitable epoxy resins include 2,2-bis[4-(2,3-oxypropoxy)phenyl]propane and 4,4'-bis(2,3-oxypropoxy)biphenyl. Specific examples include QUATREX 1010, from Dow Chemical, and ARATRONIC 5046, from Ciba Geigy. Also, epoxy novolac resins in which R⁷, R⁹, and R¹¹ are -H, and R⁸, R¹⁰, and R¹² are the Epoxy Groups (Formula II); and cresole epoxy novolac resins, in which R⁷, R⁹, and R¹¹ are -CH₃. In general, it is preferred that k be 1 and m be 1-3 (Formula II). Examples of epoxy resins falling under Formula III are disclosed in U.S. Patent No. 4,707,534.

[0068] Another suitable and preferred class of crosslinkable resins are cyanate esters. Examples of suitable cyanate esters include those having Formulas IV, V, and VI, with cyanate esters of Formulas IV and V preferred. It is preferred that the cyanate ester is present at about 25 to 75 percent by weight, more preferably about 40 to 65 percent by weight.

[0069] More generally than those cyanate ester resins falling within Formulas IV, V, and VI, cyanate ester monomers suitable for use in the present invention are cyanate esters which contain 2 or more -OCN groups, and are of the general formula:

        Q(OCN)_p     Formula IX

wherein p can be an integer from 2 to 7, and wherein Q comprises at least one of: 1) a mono-, di-, tri-, or tetrasubstituted aromatic hydrocarbon containing 5 to 30 carbon atoms; 2) 1 to 5 aliphatic or polycyclic aliphatic mono-, di-, tri-, or tetrasubstituted hydrocarbons containing 7 to 20 carbon atoms; and 3) 1 to 10 heteroatoms selected from non-peroxidic oxygen, sulfur, non-phosphino phosphorus, non-amino nitrogen, halogen, and silicon.

[0070] In the practice of this invention, a combination of cyanate ester monomers can be used whereby such combination is comprised of one or more cyanates of Formula IX and oligomers thereof, where p is 2 to 7.

[0071] Examples of cyanates are as follows: 1,3- and 1,4-dicyanatobenzene; 2-tert-butyl-1,4-dicyanatobenzene; 2,4-dimethyl-1,3-dicyanatobenzene; 2,5-di-tert-butyl-1,4-dicyanatobenzene; tetramethyl-1,4-dicyanatobenzene; 4-chloro-1,3-dicyanatobenzene; 1,3,5-tricyanatobenzenc; 2,2'- or 4,4'-dicyanatobiphenyl; 3,3',5,5'-tetramethyl-4,4'-dicyanatobiphenyl; 1,3-, 1,4-, 1,5-, 1,6-, 1,8-, 2,6-, or 2,7-dicyanatonaphthalene; 1,3,6-tricyanatonaphthalene; bis(4-cyanatophenyl)methane; bis(3-chloro-4-cyanatophenyl)methane; 2,2-bis(4-cyanatophenyl)propane; 2,2-bis(3,5-dibromo-4-cyanatophenyl)propane; 2,2-bis(4-cyanatophenyl)-1,1,1,3,3,3-hexafluoropropane; bis(4-cyanatophenyl) ether; bis(4-cyanatophenoxyphenoxy)benzene; bis(4-cyanatophenyl)ketone; bis(4-cyanatophenyl)thioether; bis(4-cyanatophenyl)sulfone; tris(4-cyanatophenyl)phosphite; and tris(4-cyanatophenyl)phosphate.

[0072] Also useful are cyanic acid esters derived from phenolic resins (e.g., disclosed in U.S. Patent No. 3,962,184), cyanated novolac resins derived from novolac (U.S. Patent No. 4,022,755), cyanated bisphenol-type polycarbonate oligomers derived from bisphenol-type polycarbonate oligomers (U.S. Patent No. 4,026,913), cyanato-terminated polyaryleneethers (U.S. Patent No. 3,595,000), dicyanate esters free of ortho hydrogen atoms (U.S. Patent No. 4,740,584), mixtures of di-and tricyanates (U.S. Patent No. 4,709,008), polyaromatic cyanates containing polycyclic aliphatic groups such as QUATREX™ 7187 from Dow Chemical Co. (U.S. Patent No. 4,528,366), fluorocarbon cyanates (U.S. Patent No. 3,733,349), and other novel cyanate compositions (U.S. Patents 4,195,132 and 4,116,946).

[0073] Polycyanate compounds obtained by reacting a phenol-formaldehyde precondensate with a halogenated cyanide are also useful.

[0074] Examples of preferred cyanate ester compositions include low molecular weight oligomers (M.W. about 250-1200) of bisphenol A dicyanates such as AROCY B-30 Cyanate Ester Semisolid Resin commercially available from Hi-Tek Polymers, Jeffersontown, Kentucky; low molecular weight oligomers of tetra o-methyl bisphenol F dicyanates, such as AROCY M-30 Cyanate Ester Semisolid Resin, commercially available from Hi-Tek Polymers; low molecular weight oligomers of thiodiphenol dicyanates, such as AROCY T-30 Cyanate Ester Semisolid Resin, commercially available from Hi-Tek Polymers; and low molecular weight oligomers of fluorinated bisphenol A dicyanates such as AROCY F-40S Cyanate Ester Resin Solution commercially available from Hi-Tek Polymers.

Catalysts and Curatives

[0075] The use of a suitable catalyst provides reaction of the crosslinkable resin in a satisfactorily short length of time. A catalyst and curative can be selected by one of ordinary skill in the art for the reaction of the particular component or components of the crosslinkable resin.

[0076] For example, suitable catalysts and curatives for reaction of epoxy resins are well known in the art. Examples of suitable curatives include aliphatic and aromatic primary or secondary amines as, for example bis(4-aminophenyl)sulfone, bis(4-aminophenyl)ether, and 2,2-bis(4aminophenyl)propane; and aliphatic and aromatic tertiary amines, such as dimethylaminopropylamine and pyridine. Examples of suitable catalysts include: boron trifluoride complexes, such as BF₃-diethylether and BF₃-monoethanolamine; imidazoles, such as 2-ethyl-4-methylimidazole; hydrazides, such as adipodihydrazide; guanidines, such as tetramethyl guanidine; and dicyandiamide.

[0077] Also useful as catalysts or supplementary catalysts are Lewis acids such as aluminum chloride, aluminum bromide, boron trifluoride, antimony pentafluoride, phosphorous pentafluoride, titanium tetrafluoride and the like. It is also desirable at times that these Lewis acids be blocked to increase the latency of compositions containing them. Representative of blocked Lewis acids are BF₃-monoethylamine and the adduct of HSbF₅X in which X is OH, a halogen, or OR²⁹ in which R²⁹ is an aliphatic or aromatic group with aniline or a hindered amine, as is described in U.S. Patent No. 4,503,211.

[0078] Additional catalysts include the imidazole-based catalysts described in U.S. Patent No. 4,331,582, such as imidazole, 2-methylimidazole; 2-ethyl-4-methylimidazole; 2-phenylimidazole; 1-benzyl-2-methylimidazole; 2-phenyl-4,5-dihydroxymethylimidazole; 2-phenyl-4-methylimidazole; 1-cyanoethyl-2-methylimidazole; 1-cyanoethyl-2-phenylimidazole.

[0079] Catalysts specific for the amine curative to epoxy reaction are described in U.S. Patent No. 4,331,582. This patent describes the use of a half-salt of a strong aromatic sulfonic acid such as toluenesulfonic acid and an imidazole, preferably an alkylated imidazole such as 2-ethyl-4-methylimidazole. In addition to the aromatic sulfonic acids mentioned in the patent, methanesulfonic acids such as trifluoromethanesulfonic acid can be used.

[0080] Catalysts for the reaction of the cyanate ester include organometallic compounds containing a cyclopentadienyl group, C₅H₅ and suitable derivatives, such as cyclopentadienyl iron dicarbonyl dimer [C₅H₅ Fe(CO)₂]₂, pentamethylcyclopentadienyl iron dicarbonyl dimer [C₅(CH₃)₅Fe(CO)₂]₂ and methylcyclopentadienyl manganese tricarbonyl C₅H₄CH₃Mn(CO)₃ which are available from Strem Chemical Company (Newburyport, MA).

Coupling Agents

[0081] The adhesive composition of the present invention may also include a coupling agent to aid in adhesion of the adhesive to a given substrate.

[0082] A silane coupling agent is preferred, and is preferably provided in an amount of about 0.1 to 5 percent by weight of the composition, more preferably about 1 percent by weight. A silane coupling agent should include a silane group at one end of the molecule and a group reactive with the crosslinkable resin at the other end.

[0083] Preferred silane coupling agents have the formula:

        P_(4-n)SiZ_n

in which:

P represents an organic substituent of up to 12 carbon atoms; e.g., propyl, which should possess functional substituents such as mercapto, epoxy, glycidoxy, acrylyl, methacrylyl and amino;

Z represents a hydrolyzable group, e.g., alkoxy, preferably methoxy or ethoxy, and

n is 1, 2, or 3, preferably 3.

[0084] In addition to silane coupling agents, titanate and zirconate coupling agents may be utilized. Reference is made to the chapter on coupling agents by J.Cromyn in Structural Adhesives edited by A.J.Kinloch, published by Elsevier Applied Science Publishers, 1986, pp269-312, Page 270 provides examples of epoxy and amine silane coupling agents. Pages 306-308 discuss the use of titanate and zirconate coupling agents.

Additional Additives

[0085] Conductive particles, including, for example, those described in U.S. Patent No. 4,606,962 (Reylek et al.), may be added to the adhesive composition as desired. Reylek describes electrically and thermally conductive particles that at the bonding temperature of the adhesive are at least as deformable as are substantially pure silver spherical particles. The thickness of the particles exceeds the thickness of the adhesive between the particles. The particles described in Reylek are preferably substantially spherical and made of a metal such as silver or gold, or of more than one material, such as "a solder surface layer and either a higher melting metal core such as copper or a non-metallic core" (column 4, lines 20-21). These and other conductive particles (e.g., non-spherical and/or having a thickness of less than the thickness of the adhesive) are suitable for use in the adhesive composition of the present invention. Conductive particles contained in adhesive compositions of the invention may be randomly dispersed therein or may be arranged in a uniform array of desired configuration.

[0086] Other fillers can be added to the adhesive composition. The use of a filler can provide benefits of increased adhesion, higher modulus, and decreased coefficient of thermal expansion. Useful fillers include, but are not limited to, the following: silica particles, silicate particles, quartz particles, ceramics particles, glass bubbles, inert fibers, and mica particles. Preferably, the filler is microcrystalline silica particles.

[0087] It is important that the adhesive composition have a low ionic impurity level. The electronics industry specifies low extractable ion content of the adhesives. Specifications include Cl^-, Na⁺, K⁺, and NH₄⁺ of less than 10 parts per million by weight (ppm). Such extremely low ionic contents are important to prevent corrosion of the metals and to keep the conductivity of the adhesive as low as possible except through any conductive metal particles present.

Examples

[0088] The invention will be further explained by the following illustrative examples which are intended to be nonlimiting. All amounts are expressed in parts by weight unless otherwise indicated. When used herein R.Ex means Reference Example.

Reference Examples 1,3-4 and 6 and Examples 2 and 5

[0089] An adhesive composition of the present invention was made by mixing together 93 parts of P1700-NT11, a polysulfone resin from Amoco Performance Products (hereinafter PSU), 55 parts of QUATREX 1010, an electronic grade diglycidyl ether of bisphenol A resin from the Dow Chemical Company, (hereinafter Q1010), and 38 parts of 9,9-bis(3-methyl-4-aminophenyl)fluorene (hereinafter BAFOT). These three constituents were stirred to a uniform paste, using 180 parts of methylene chloride as a solvent for the solution of the PSU and Q1010 resins, followed by blending in the relatively insoluble BAFOT. The BAFOT had been jet-milled to a mean particle size of less than about 5 microns (µm). To this dispersion, 4 parts of an accelerator, the trifluoromethanesulfonic acid half salt of 2-ethyl-4-methylimidazole (hereinafter 2E4MI 1/2T), and 1.88 parts of a silane coupling agent, (3-glycidoxypropyl)trimethoxysilane (hereinafter 3-GPMS), were added. The epoxy adhesive system was coated onto polyester film using a knife coater set at 51 µm and dried overnight at room temperature to give a dry film 20 µm thick.

[0090] The active face of a chip was bonded to a glass substrate using the following method. A piece of the dry adhesive film was placed backing side down on a hotplate set at 120°C and a bumped silicon test chip, having a 0.2 mm (3M120 test chip) or 0.1 mm (3M328 test chip) center to center spacing between the leads, was placed on the adhesive with the active side of the chip contacting the adhesive. Gentle pressure was applied to the chip to insure proper tacking of the adhesive. The adhesive and chip were removed from the hotplate and allowed to cool. The adhesive was scored around the perimeter of the chip and the backing was removed along with the excess adhesive, leaving the active face of the chip completely covered with adhesive. The chip was checked to be sure that the bumps had pierced through the adhesive. The chip was bonded to a glass test substrate having ITO or nickel leads. The bonding was accomplished at 180°C with a bonding time of 3 minutes. For each 3M120 bumped test chip a bonding force equivalent to approximately 375 grams (g) per bump was applied, and for each 3M328 bumped chip a bonding force equivalent to 82 g per bump was applied. These bonding forces corresponded to a compressive stress of approximately 345 MPa applied to each bump. The chip was removed from the glass by shearing the chip at 180°C and the remaining adhesive was removed from the glass by rubbing a cotton swab soaked in methylene chloride over the glass. A fresh chip was bonded to the glass and excellent interconnection was again achieved.

[0091] Samples were also made by bonding unbumped 3M120 chips with adhesive having 20 µm Bell Pearl (nickel coated phenoxy resin) conductive particles embossed into the adhesive in places corresponding to the pads on the chip and substrate. Bonding was accomplished in the same manner as described above except that an applied force of 225 g per pad was used. This bonding force corresponded to a compressive stress of approximately 210 MPa applied to each pad.

[0092] Environmental aging tests were performed on samples prepared and bonded in the manner described above. The bonded samples were tested by subjecting a sample to humidity aging at 85°C at 85 percent relative humidity, a sample to thermal aging at 125°C, and a sample to thermal cycling from -55°C to 125°C. Each test was run for 1000 hours. The results are summarized Table 1.

Table 1

R.Ex and		Pad Spacing	Chip	85°C/85% RH	-55 to 125°C	125°C
Ex.	Leads
1(R.Ex)	ITO	0.2 mm	bumped	pass	pass	pass
2	ITO	0.2 mm	unbumped	failed	pass	pass
3(R.Ex)	ITO	0.1 mm	bumped	pass	pass	pass
4(R.Ex)	Ni	0.2 mm	bumped	Ni corrosion	pass	pass
5	Ni	0.2 mm	unbumped	Ni corrosion	pass	pass
6(R.Ex)	Ni	0.1 mm	bumped	Ni corrosion	pass	pass

[0093] To pass the test interconnections must have been maintained for the duration of the test. The failure shown for Example 2 at 85°C/85 percent relative humidity may have been caused by a contamination of the deionized water supply. The nickel corrosion shown in the table was not caused by the adhesive. The corrosion occurred outside of the area covered by adhesive and was caused by the humidity in the chamber. Figures 3 and 4 show graphs of the thermal cycling (samples A-F) and thermal aging (samples G-L), respectively, for the above Examples and Reference Examples.

Example 7

[0094] An adhesive composition of the present invention was made by adding 20.6 parts of FINEPEARL™ (gold on nickel on polystyrene having an average diameter of 6 µm, from Sumitomo Chemical, Osaka, Japan) conductive particles to the adhesive formulation of Examples and Reference Examples 1-6 by dispersing the particles in the adhesive mixture before the adhesive was knife-coated. The adhesive with particles was coated onto a polyester film using a knife coater set at 76 µm and dried overnight at room temperature to give a dry film 30 µm thick. The adhesive was used to bond a polyimide flexible printed circuit (FPC) to a FR4 printed circuit board. The FPC comprises a 50 µm polyimide backing having 17 stripes with a spacing of 0.4 mm center to center and made of 28-39 (1 ounce) copper coated with 13µm solder. The printed circuit board comprises 28-39 (1 ounce) copper stripes coated with 13 µm solder with 0.4 mm center to center spacing on a FR4 laminate board. Before bonding, a piece of adhesive with backing was cut to measure 3.2 mm by 7.9 mm.

[0095] The adhesive was pretacked to the FPC by heating the FPC to 110°C and pressing the adhesive strip with the backing side up onto the FPC across the stripes using slight pressure. After cooling the backing was peeled off. The FPC was placed on the printed circuit board with the adhesive facing the board. The stripes on the FPC were aligned with the stripes on the board. Bonding was accomplished using a Scotchlink bonder having a 3.2 mm bare steel bar set at 217°C and bonding for 3 minutes under 1.7 to 2.1 MPa. The FPC could be removed from the circuit board by heating the board and pulling off the FPC. Both the FPC and the board could be cleaned using methylene chloride and reused. Samples bonded in the manner described above passed 1000 hours of environmental aging at 60°C/95 percent relative humidity aging, 60°C/95 percent relative humidity/15 V biased humidity aging, 85°C/85 percent relative humidity aging, -55 to 125°C thermal cycling, -40°C to 105°C thermal cycling, 100°C thermal aging, and 125°C thermal aging. To pass the test interconnection resistance could not exceed 1 ohm at the end of the test. Figure 5 shows graphs of the four most rigorous of these tests (samples M-P).

Example 8

[0096] An adhesive composition of the present invention was made by dissolving 20 parts of PSU in 10 parts of tetrahydrofuran (hereinafter THF). To this solution, 20 parts of 9,9-bis[4-(2,3-epoxypropoxy)phenyl]fluorene (FEP) were added with stirring until the FEP dissolved. More THF was added as needed to dissolve the FEP. After all of the FEP was dissolved, the solution was stirred uncovered until enough THF had evaporated to give a solution with a viscosity suitable for making a film. At this point a solution of 1 part of 2-methylimidazole (hereinafter 2MZ) dissolved in a minimum of THF and 0.48 part of 3-GPMS were stirred into the PSU and FEP mixture. After 4.56 parts of FINEPEARL™ conductive particles were added, the mixture was stirred to a uniform dispersion and knife-coated onto polyester film using a gap of 102 µm. The film was allowed to dry overnight at room temperature to give a dry film 30 µm thick. The adhesive was bonded as in the previous example except that the bonder was set at 237°C and the bonding time was 30 seconds. As above, the FPC was removed from the circuit board by heating the board and pulling off the FPC. Both the FPC and the board could be cleaned using methylene chloride and reused. Samples bonded in the manner described above passed all of the environmental tests described in Example 7 with all of the resistances under 1 ohm. Figure 6 shows graphs of the four most rigorous of these tests (samples Q-T).

Reference

Example 9

[0097] Reworkability depends on the amount of thermoplastic in the formulation. When the adhesive from R.Ex 1 was made with varying amounts of PSU and used to bond a bumped 3M120 test chip to a glass substrate using the bonding conditions of Example 1, the reworkability was tested by measuring the force needed to shear the bond, with solvents used to remove the remaining adhesive from the glass. When 39 parts of PSU were used, 1.8 MPa of force were needed to shear the chip from the substrate at 230°C, and the adhesive residue could not be removed with solvent. When 55.8 parts of PSU were used, the bond could be sheared at 180°C using 2.1 MPa of force, but not all of the adhesive could be removed with solvent. When 74.4 parts of PSU were used, 0.81 MPa of force were needed to shear the bond at 185°C, and all of the adhesive was removed by scrubbing with a swab saturated with methylene chloride. When 93 parts of PSU were used as in R.Ex 1, 0.22 MPa of force was needed to remove the chip, and all of the adhesive was removed by scrubbing with a swab saturated with methylene chloride. Thus, in this embodiment more than 55.8 parts of PSU are needed for reworkability.

Reference

Example 10

[0098] The influence of bonding time on reworkability was examined. Samples of the adhesive used in R.Ex 1 were tacked to glass slides and placed in an oven at 180°C for 3, 4 and 5 minutes. After cooling, the slides were placed in beakers of methylene chloride. The sample cured for 3 minutes dissolved rapidly. The sample cured for 4 minutes appeared to dissolve somewhat, but not completely. When tweezers were used to pull the film off the slide, the film curled into a ball. After 3 hours the adhesive had not dissolved. The sample cured for 5 minutes did not dissolve and could be peeled off the slide in one piece. After 3 hours the adhesive film had not dissolved. When bonds were made as before, except with 4 and 5 minute bonding times, the bonds could be sheared at 180°C with less than 0.22 MPa of force for the 4 minute bond, and less than 0.33 MPa for the 5 minute bonds. The adhesive could be removed from the glass substrates and the chips by scrubbing with a cotton swab saturated with methylene chloride.

Example 11

[0099] An adhesive of the present invention was prepared by mixing the following components:

Component	Parts
polyvinyl butyral, S-LEC BX-L, Sekisui Chemical Company	13.32
cyanate ester, AROCY B-30, Hi-Tek Polymers	13.32
cyclopentadienyl iron dicarbonyl dimer	0.05
3-GPMS	0.30
FINEPEARL™	3.00
THF	70.01

[0100] The polyvinyl butyral and the cyanate ester were dissolved in the THF. The 3-GPMS, the cyclopentadienyl iron dicarbonyl dimer catalyst, and the conductive particles were added and mixed into the solution. The adhesive composition was cast onto the polyester film using a knife coater to provide a dry film thickness of 35 µm. The polyester film had a silicone release coating to facilitate the release of the adhesive. The adhesive film was dried at room temperature for about 24 hours. The adhesive was bonded as in example 7 except that the bonding time was 2 minutes. As in example 7, the FPC was removed from the circuit board, and both the FPC and the board could be cleaned and reused. Bonded samples were subjected to the same environmental testing as done in example 7 with all samples passing the tests and having resistances under 0.3 ohm.

[0101] Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope of this invention.

Claims

1. An adhesive composition comprising a blend of:

a) a thermoplastic polymer having a T_g of 30°C to 250°C, wherein said thermoplastic polymer is selected from polysulfones, polyamides, polyesters, polycarbonates, polyvinyl acetals and polyethers;

b) a crosslinkable resin having a T_g of 125°C to 300°C;

c) a catalyst capable of accelerating the cure reaction of said crosslinkable resin, and

d) conductive particles;

wherein said adhesive composition is curable to a reworkable, electrically conductive Z-axis adhesive.

2. The reworkable adhesive composition of Claim 1 wherein said thermoplastic polymer is a polysulfone having the following general formula:

wherein m is 0 or 1, and n is 10-500.

3. The reworkable adhesive composition of Claim 2 wherein m is 1 and n is 40-70 or m is 0 and n is 12-50.

4. The reworkable adhesive composition of Claim 1 wherein said thermoplastic polymer is a polyvinyl acetal having the following general formula:

   wherein X is -H, or a C₁-C₄ alkyl group, and n is 10 to 2200, m is 80 to 2400, and p is 0 to 500.

5. The reworkable adhesive composition of any preceding Claim containing a coupling agent with the following formula:

        P_(4-n)SiZ_n

in which:

P is an organic substituent of up to 12 carbon atoms;

Z is a hydrolyzable group, and

n is 1, 2, or 3.

6. The reworkable adhesive composition of any preceding Claim wherein said crosslinkable resin comprises an epoxy resin and a curative therefor, and said epoxy resin comprises 25 to 80 percent by weight of total adhesive resin.

7. The reworkable adhesive composition of any preceding Claim wherein said crosslinkable resin is an epoxy resin comprising one or more epoxy compounds according to one or more of the following Formulas I, II, and III:

   wherein X is a single bond, a C₁-C₄ alkylene group, or the -SO₂- group; and R¹, R², R³, R⁴, R⁵, R⁶ are independently H, a C₁-C₅ alkyl group, or the Epoxy Group, wherein at least two of R¹ - R⁶ are the Epoxy Group;

   wherein R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹² are independently -H, a C₁-C₅ alkyl group, or the Epoxy Group, k is 0 or 1, and m is 0 to 9, with at least two of R⁷-R¹² being the Epoxy Group, except when k is 0, wherein both R⁷ and R⁸ are the Epoxy Group; and

   where R¹³ , R¹⁴ are independently

   wherein R¹⁵, R¹⁶, R¹⁷ are independently -H, a C₁-C₅ alkyl group, or the Epoxy Group, wherein R¹³, R¹⁴ combined include two or more Epoxy Groups.

8. The reworkable adhesive composition of Claim 7 wherein in Formula I, R² and R⁵ are both the Epoxy Group, and R¹, R³, R⁴, R⁶ are independently -H, or -CH₃, X is a single bond, -CH₂-, -C(CH₃)₂-, or the -SO₂- group.

9. The reworkable adhesive composition of Claim 8 further including an epoxy curative comprising a fluorene curative according to Formula III except wherein one of R¹⁵-R¹⁷ is -NH₂ or -NHCH₃, and the other two of R¹⁵-R¹⁷ are H or a C₁-C₅ alkyl group, wherein R¹⁵ - R¹⁷ may be different for R¹³ and R¹⁴.

10. The reworkable adhesive composition of any of Claims 7 to 9 further comprising a catalyst including an imidazole ring.

11. The reworkable adhesive composition of any of Claims 1 to 5 wherein said crosslinkable resin comprises a cyanate ester resin including one or more cyanate ester compounds according to one or more of the following Formulas IV, V, and/or VI:

   wherein Y is a single bond, a C₁-C₄ alkylene group, -S-, or the -SO₂- group; and R¹⁸, R¹⁹, R²⁰, R²¹, R²², R²³ are independently -H, a C₁-C₅ alkyl group, or the Cyanate Ester Group; where at least two of R¹⁸-R²³ are the Cyanate Ester Group;

where n is 0 to 5; and

where R²⁴, R²⁵ are independently

   wherein R²⁶, R²⁷, R²⁸, are independently -H, a C₁ - C₅ alkyl group, or the Cyanate Ester Group, where R²⁴, R²⁵ combined include two or more Cyanate Ester Groups, and where R²⁶-R²⁸ may be different for R²⁴ and R²⁵, said cyanate ester resin being present at 25 to 75 percent by weight of total adhesive resin.

12. The reworkable adhesive composition of Claim 11 further including an organometallic compound containing a cyclopentadienyl group as a catalyst for the reaction of said cyanate ester.

13. A method for forming a reworkable adhesive bond between two conductive surfaces to form a conductive bonded composite comprising the steps of :

a) providing a film of reworkable electrically conductive Z-axis adhesive composition as claimed in any preceding Claim;

b) providing a conductive substrate (10) having one or more conductive bonding sites (20, 30, 40);

c) providing one or more conductive devices (50, 60-66, 80, 90) each having a conductive bonding surface;

d) positioning said reworkable adhesive between each of said conductive bonding surfaces of said conductive devices; and

e) heating to a temperature of from 125°C to 250°C for from 5 to 300 seconds to form a reworkable adhesive bond between each conductive bonding site and each conductive bonding surface.

14. The method of Claim 13 wherein said conductive substrate (10) is a printed circuit board.

15. The method of Claim 13 or Claim 14 wherein said conductive bonding surface comprises a material selected from the group consisting of gold, silver, copper, aluminum, nickel, and solder.

16. An electronic assembly, comprising:

a) a first electronic component (10, 20, 30, 40, 50, 60, 61-66, 70, 80, 90, 100) with a conductive bonding site;

b) a second electronic component (10, 20, 30, 40, 50, 60, 61-66, 70, 80, 90, 100) with a conductive bonding site;

c) the adhesive composition of any of Claims 1 to 12 between a bonding site on the first component and a bonding site on the second component.

Ansprüche

1. Klebstoffzusammensetzung, umfassend ein Blend aus:

a) einem thermoplastischen Polymer mit einer T_g von 30 °C ... 250 °C, wobei das thermoplastische Polymer ausgewählt wird aus Polysulfonen, Polyamiden, Polyestern, Polycarbonaten, Polyvinylacetalen und Polyethern;

b) einem vernetzbaren Harz mit einer T_g von 125 °C ... 300 °C;

c) einem Katalysator, der die Vernetzungsreaktion des vernetzungsfähigen Harzes beschleunigen kann, und

d) leitfähigen Partikeln;

welche Klebstoffzusammensetzung zu einem wiederverarbeitbaren, elektrisch leitfähigen Z-Achsen-Klebstoff vernetzungsfähig ist.

2. Wiederverarbeitbare Klebstoffzusammensetzung nach Anspruch 1, bei welcher das thermoplastische Polymer ein Polysulfon der allqemeinen Formel ist:

worin m Null oder 1 und n 10 ... 500 ist.

3. Wiederverarbeitbare Klebstoffzusammensetzung nach Anspruch 1, bei welcher m 1 und n 40 ... 70 oder m Null ist und n 12 ... 50 ist.

4. Wiederverarbeitbare Klebstoffzusammensetzung nach Anspruch 1, bei welcher das thermoplastische Polymer ein Polvvinvlacetal der allgemeinen Formel ist:

worin X Wasserstoff oder eine C_1...4-Alkyl-Gruppe ist und n 10 ... 2.200, m 80 ... 2.400 und p Null ... 500.

5. Wiederverarbeitbare Klebstoffzusammensetzung nach einem der vorgenannten Ansprüche, enthaltend ein Kupplungsmittel der Formel:

        P_(4-n)SiZ_n

worin sind:

P ein organischer Substituent mit bis zu 12 Kohlenstoffatomen;

Z eine hydrolysierbare Gruppe und

n 1, 2 oder 3.

6. Wiederverarbeitbare Klebstoffzusammensetzung nach einem der vorgenannten Ansprüche, bei welcher das vernetzungsfähige Harz ein Epoxidharz und einen Härter dafür umfaßt und bei der Epoxidharz 25 % ... 80 Gewichtsprozent des gesamten Klebstoffharzes ausmacht.

7. Wiederverarbeitbare Klebstoffzusammensetzung nach einem der vorgenannten Ansprüche, bei welcher das vernetzungsfähige Harz ein Epoxidharz ist, umfassend eine oder mehrere Epoxy-Verbindungen entsprechend einer oder mehrerer der folgenden Formeln I, II und III:

worin X eine Einfachbindung ist, eine C_1...4-Alkylen-Gruppe oder die -SO₂-Gruppe; und R¹, R², R³, R⁴, R⁵, R⁶ unabhängig Wasserstoff sind, eine C_1...4-Alkyl-Gruppe oder die Epoxy-Gruppe, wobei mindestens zwei von R¹ ... R⁶ die Epoxy-Gruppe sind:

worin R⁷, R⁸, R⁹, R¹⁰, R¹¹, R¹² unabhängig Wasserstoff, eine C_1...5-Alkyl-Gruppe oder die Epoxy-Gruppe sind, k ist Null oder 1 und m Null ... 9, wobei mindestens zwei der R⁷ ... R¹² die Epoxy-Gruppe sind, ausgenommen wenn k Null ist, wobei sowohl R⁷ als auch R⁸ die Epoxy-Gruppe sind; und

darin sind R^13, R¹⁴ unabhängig

worin R¹⁵, R¹⁶, R¹⁷ unabhängig Wasserstoff, eine C_1...5-Alkyl-Gruppe oder die Epoxy-Gruppe sind, wobei R¹³ und R¹⁴ vereinigt zwei oder mehrere Epoxy-Gruppen sind.

8. Wiederverarbeitbare Klebstoffzusammensetzung nach Anspruch 7, bei welcher in Formel I R² und R⁵ beide die Epoxy-Gruppe sind, R¹, R³, R⁴, R⁶ sind unabhängig Wasserstoff oder -CH₃, X ist eine Einfachbindung, -CH₂-, -C(CH₃)₂- oder die -SO₂- Gruppe.

9. Wiederverarbeitbare Klebstoffzusammensetzung nach Anspruch 8, ferner einschließend ein Epoxy-Härter, umfassend ein Fluoren-Härter entsprechend Formel III mit der Ausnahme, wenn eines von R¹⁵ ... R¹⁷ -NH₂ oder -NHCH₃ ist, und die anderen zwei von R¹⁵ ... R¹⁷ Wasserstoff sind, oder eine C_1...5-Alkyl-Gruppe , bei der R¹⁵ ... R¹⁷ bei R¹³ und R¹⁴ verschieden sein können.

10. Wiederverarbeitbare Klebstoffzusammensetzung nach einem der Ansprüche 7 bis 9, ferner umfassend, einen Katalysator, der einen Imidazol-Ring einschließt.

11. Wiederverarbeitbare Klebstoffzusammensetzung nach einem der Ansprüche 1 bis 5, bei welcher das vernetzungsfähige Harz ein Cyanatester-Harz umfaßt, einschließend eine oder mehrere Cyanatester-Verbindungen entsprechend einer oder mehreren der folgenden Formeln IV, V und/oder VI:

worin Y eine Einfachbindung ist, eine C_1...4-Alkylen-Gruppe, -S- oder die -SO₂-Gruppe; und R¹⁸, R¹⁹, R²⁰, R²¹, R²², R²³ unabhängig Wasserstoff sind, eine C_1...5-Alkyl-Gruppe oder die Cyanatester-Gruppe; wobei mindestens zwei von R¹⁸ ... R²³ die Cyanatester-Gruppe sind;

worin n Null ... 5 ist, und

worin R²⁴,R²⁵ unabhängig

sind;
worin R²⁶, R²⁷, R²⁸ unabhängig Wasserstoff, eine C_1...5-Alkyl-Gruppe oder die Cyanatester-Gruppe sind, wobei R²⁴, R²⁵ vereint zwei oder mehrere Cyanatester-Gruppen einschließen und wobei R²⁶ ... R²⁸ bei R²⁴ und R²⁵ verschieden sein können, wobei das Cyanatester-Harz mit 25 % ... 75 Gewichtsprozent des gesamten Klebstoffharzes vorliegt.

12. Wiederverarbeitbare Klebstoffzusammensetzung nach Anspruch 11, ferner einschließend eine metallorganische Verbindung, enthaltend eine Cyclopentadienyl-Gruppe als einen Katalysator für die Reaktion des Cyanatesters.

13. Verfahren zum Erzeugen einer wiederverarbeitbaren Klebverbindung zwischen zwei leitfähigen Oberflächen zur Erzeugung eines leitfähig verklebten Verbundstoffes, umfassend die Schritte:

a) Bereitstellen eines Films einer wiederverarbeitbaren, elektrisch leitfähigen Z-Achsen-Klebstoff-zusammensetzung nach einem der vorgenannten Ansprüche;

b) Bereitstellen eines leitfähigen Substrats (10) mit einer oder mehreren leitfähigen Stellen zum Bonden (20, 30, 40);

c) Bereitstellen einer oder mehrerer leitfähiger Vorrichtungen (50, 60 ... 66, 80, 90), von denen jede über eine leitfähige Oberfläche zum Bonden verfügt;

d) Aufbringen des wiederverarbeitbaren Klebstoffes zwischen jeder der leitfähigen Oberflächen zum Bonden der leitfähigen Vorrichtungen; sowie

e) Erhitzen auf eine Temperatur von 125 °C ... 250 °C für 5 .. 300 Sekunden, um eine wiederverarbeitbare Klebverbindung zwischen jeder der leitfähigen Stellen zum Bonden und jeder leitfähigen Oberfläche zum Bonden zu erzeugen.

14. Verfahren nach Anspruch 13, bei welchem das leitfähige Substrat (10) eine Leiterplatte ist.

15. Verfahren nach Anspruch 13 oder 14, bei welchem die leitfähige Oberfläche zum Bonden ein Material umfaßt, ausgewählt aus der Gruppe, bestehend aus Gold, Silber, Kupfer, Aluminium, Nickel und Lot.

16. Elektronische Baugruppe, umfassend:

a) ein erstes elektronisches Bauelement (10, 20, 30, 40, 50, 60, 61 ... 66, 70, 80, 90, 100) mit einer leitfähigen Stelle zum Bonden;

b) ein zweites elektronisches Bauelement (10, 20, 30, 40, 50, 60, 61 ... 66, 70, 80, 90, 100) mit einer leitfähigen Stelle zum Bonden;

c) eine Klebstoffzusammensetzung nach einem der Ansprüche 1 bis 12 zwischen einer Stelle zum Bonden auf dem ersten Bauelement und einer Stelle zum Bonden auf dem zweiten Bauelement.

Revendications

1. Composition d'adhésif comprenant un mélange de :

a) un polymère thermoplastique ayant une T_g de 30°C à 250°C, ledit polymère thermoplastique étant choisi parmi les polysulfones, les polyamides, les polyesters, les polycarbonates, les polyvinylacétals et les polyéthers ;

b) une résine réticulable ayant une T_g de 125°C à 300°C ;

c) un catalyseur capable d'accélérer la réaction de durcissement de ladite résine réticulable ; et

d) des particules conductrices ;

ladite composition d'adhésif pouvant être durcie en un adhésif électroconducteur recyclable à axe z.

2. Composition d'adhésif recyclable selon la revendication 1, dans laquelle ledit polymère thermoplastique est une polysulfone ayant la formule générale suivante :

   où m vaut 0 ou 1, et n vaut de 10 à 500.

3. Composition d'adhésif recyclable selon la revendication 2, dans laquelle m vaut 1 et n vaut de 40 à 70, ou m vaut 0 et n vaut de 12 à 50.

4. Composition d'adhésif recyclable selon la revendication 1, dans laquelle ledit polymère thermoplastique est un polyvinylacétal ayant la formule générale suivante :

   où X est -H ou un groupe alkyle en C₁-C₄, et n vaut de 10 à 2 200, m vaut de 80 à 2 400, et p vaut de 0 à 500.

5. Composition d'adhésif recyclable selon l'une quelconque des revendications précédentes, contenant un agent de couplage ayant la formule suivante :

        P_(4-n)SiZ_n

où

P est un substituant organique ayant jusqu'à 12 atomes de carbone ;

Z est un groupe hydrolysable ; et

n vaut 1, 2 ou 3.

6. Composition d'adhésif recyclable selon l'une quelconque des revendications précédentes, dans laquelle ladite résine réticulable comprend une résine époxy et un agent de durcissement pour celle-ci, et ladite résine époxy représente de 25 à 80 % en poids de ladite résine totale de l'adhésif.

7. Composition d'adhésif recyclable selon l'une quelconque des revendications précédentes, dans laquelle ladite résine réticulable est une résine époxy comprenant un ou plusieurs composés époxy selon l'une ou plusieurs des formules I, II et III suivantes :

   où X est une liaison simple, un groupe alkylène en C₁-C₄ ou le groupe -SO₂- ; et R¹, R², R³, R⁴, R⁵ et R⁶ sont indépendamment H, un groupe alkyle en C₁-C₅ ou le groupe époxy, au moins deux radicaux R¹ à R⁶ signifiant le groupe époxy ;

   où R⁷, R⁸, R⁹, R¹⁰, R¹¹ et R¹² sont indépendamment -H, un groupe alkyle en C₁-C₅ ou le groupe époxy, k vaut 0 ou 1, et m vaut de 0 à 9, au moins deux radicaux R⁷ à R¹² signifiant le groupe époxy, sauf quand k vaut 0, auquel cas R⁷ et R⁸ signifient tous les deux le groupe époxy ; et

   où R¹³ et R¹⁴ représentent indépendamment

   où R¹⁵, R¹⁶ et R¹⁷ sont indépendamment -H, un groupe alkyle en C₁-C₅ ou le groupe époxy, où R¹³ et R¹⁴ combinés comprennent au moins deux groupes époxy.

8. Composition d'adhésif recyclable selon la revendication 7, dans laquelle, dans la formule I, R² et R⁵ signifient chacun le groupe époxy, et R¹, R³, R⁴ et R⁶ signifient indépendamment -H ou -CH₃, X est une liaison simple, -CH₂-, -C(CH₃)₂- ou le groupe -SO₂-.

9. Composition d'adhésif recyclable selon la revendication 8, comprenant, en outre, un agent de durcissement de l'époxy comprenant un agent de durcissement fluorénique selon la formule III sauf qu'un des radicaux R¹⁵ à R¹⁷ est -NH₂ ou -NHCH₃, et les deux autres radicaux R¹⁵-R¹⁷ signifient H ou un groupe alkyle en C₁-C₅, R¹⁵ à R¹⁷ pouvant être différents pour R¹³ et R¹⁴.

10. Composition d'adhésif recyclable selon l'une quelconque des revendications 7 à 9, comprenant, en outre, un catalyseur comprenant un noyau imidazole.

11. Composition d'adhésif recyclable selon l'une quelconque des revendications 1 à 5, dans laquelle ladite résine réticulable comprend une résine d'ester cyanate comprenant un ou plusieurs composés esters cyanates selon l'une ou plusieurs des formules IV, V et/ou VI suivantes ;

   où Y est une liaison simple, un groupe alkylène en C₁-C₄, -S- ou le groupe -SO₂- ; et R¹⁸, R¹⁹, R²⁰, R²¹, R²² et R²³ signifient indépendamment -H, un groupe alkyle en C₁-C₅ ou le groupe ester cyanate ; au moins deux des radicaux R¹⁸ à R²³ signifiant le groupe ester cyanate ;

où n vaut de 0 à 5 ; et

où R²⁴ et R²⁵ sont indépendamment

   où R²⁶, R²⁷ et R²⁸ signifient indépendamment -H, un groupe alkyle en C₁-C₅ ou le groupe ester cyanate, où R²⁴ et R²⁵ combinés comprennent au moins groupes esters cyanates, et où R²⁶ à R²⁸ peuvent être différents pour R²⁴ et R²⁵, ladite résine d'ester cyanate étant présente en une quantité de 25 à 75 % en poids par rapport à la résine totale de l'adhésif.

12. Composition d'adhésif recyclable selon la revendication 11, comprenant, en outre, un composé organométallique contenant un groupe cyclopentadiényle comme catalyseur pour la réaction dudit ester cyanate.

13. Procédé de formation d'une liaison d'adhésif recyclable entre deux surfaces conductrices pour former un assemblage composite conducteur, comprenant les étapes consistant à :

a) fournir un film d'une composition d'adhésif électroconducteur recyclable à axe z telle que revendiquée dans l'une quelconque des revendications précédentes ;

b) fournir un substrat conducteur (10) ayant un ou plusieurs sites conducteurs d'assemblage (20, 30, 40) ;

c) fournir un ou plusieurs dispositifs conducteurs (50, 60-66, 80, 90) ayant chacun une surface conductrice d'assemblage ;

d) positionner ledit adhésif recyclable entre chacune desdites surfaces conductrices d'assemblage desdits dispositifs conducteurs ; et

e) effectuer un chauffage à une température de 125°C à 250°C pendant 5 à 300 secondes pour former une liaison d'adhésif recyclable entre chaque site conducteur d'assemblage et chaque surface conductrice d'assemblage.

14. Procédé selon la revendication 13, dans lequel ledit substrat conducteur (10) est une plaquette à circuit imprimé.

15. Procédé selon la revendication 13 ou la revendication 14, dans lequel ladite surface conductrice d'assemblage comprend un matériau choisi dans l'ensemble constitué par l'or, l'argent, le cuivre, l'aluminium, le nickel et la soudure.

16. Assemblage électronique comprenant ;

a) un premier composant électronique (10, 20, 30, 40, 50, 60, 61-66, 70, 80, 90, 100) avec un site conducteur d'assemblage ;

b) un second composant électronique (10, 20, 30, 40, 50, 60, 61-66, 70, 80, 90, 100) avec un site conducteur d'assemblage ;

c) la composition d'adhésif selon l'une quelconque des revendications 1 à 12 entre un site d'assemblage sur le premier composant et un site d'assemblage sur le second composant.

Drawing